This invention relates to abrasive particles consisting essentially of titanium carbide, zirconium carbide and titanium diboride and a process for producing these particles.
Abrasive particles are incorporated into grinding wheels, cutting wheels and abrasive belts to grind or cut metals and other hard materials. Such wheels, belts and the like are judged by their ability to grind or cut rapidly with a minimum of applied force, with long service life and with the ability to produce a smooth, uniform surface with a minimum of structural damage. Desirable performance of these wheels, belts and the like are attributed to the abrasive particle which must possess great hardness and chemical inertness toward the material being ground, but other factors are also important as will be discussed below. The conventional abrasive particles of the trade are aluminum oxide and silicon carbide. These materials are inexpensive, but in the case of aluminum oxide, wear out rapidly or, in the case of silicon carbide, react when used on most steels. For certain applications expensive high performance abrasive particles such as diamond and cubic boron nitride are used. These materials have a very long service life, i.e., wear out very slowly but cost approximately ten thousand times more than conventional abrasives.
It is an object of this invention to provide abrasive particles which give performance approaching that of diamond and cubic boron nitride at a lesser cost.
Transition metal carbides, well-known for their great hardness and high melting points, are widely used in commercial applications such as cutting tools and dies, usually with a ductile metal binder. Although these carbides have great hardness and high melting points, repeated attempts to use them as abrasives have shown that they do not compete successfully with conventional abrasives such as aluminum oxide, particularly in the grinding of ferrous metals (L. Coes, Jr., Abrasives, Springer-Verlag, New York - Wien, 1971, pp. 114-116 and NSF Hard Materials Research, Volume 1, page 92, Carnegie-Mellon University Section, Pennsylvania State University 1972).
Some improvements in the physical properties, particularly heat conductivity and toughness of transition metal carbide composites used for cutting tools and wear resistant surfaces, have been achieved by combining one or more transition metal carbides with boron or transition metal borides as described in Glaser U.S. Pat. Nos. 2,806,800 and 2,814,566, Williams U.S. Pat. No. 3,497,368 and Schedler Austrian Pat. No. 199,886. These improvements are generally directed to cutting tool applications where hardness to resist abrasive wear and very high toughness to resist shock loading are required. Cutting tool art has shown that these properties are optimized by fine microstructural grain size (0.1.mu.m or less). On the other hand, an abrasive particle having the grinding characteristics of a high performance abrasive, specifically sharpness and long life, requires a combination of hardness with moderate toughness. The abrasive particle will have long-lived sharp cutting edges that predictably break down by fracture to give fresh cutting edges rather than rounded ones. If the toughness is too low, the particle is brittle and breaks down too quickly in abrasive use. Examples of this are pure titanium carbide and zirconium carbide which are hard but wear too fast. Essentially an abrasive particle must be friable to a certain controlled degree, a property of a very limited range of hard chemical compositions and apparently also promoted by a relatively coarse grain structure. The major chemical constituents of the abrasive particles should be nonreactive at the grinding temperature with the metals being ground. Thus while the above patents describe composites of a very wide range of transition metal carbides and borides including those of titanium, molybdenum, tungsten, iron, manganese, chromium and silicon, they in no way teach the limited range of compositions or the process techniques required to produce the high performance abrasive of this invention.
The objective of this invention is met by abrasive particles which consist essentially of a mixed carbide matrix of titanium carbide and zirconium carbide, at least partially in solid solution form and having a crystalline or grain size of about 2 to 30.mu.m, and crystals of titanium diboride, 0.5.mu.m to 30.mu.m in size, dispersed throughout the carbide matrix.
It will be understood that the presence or absence of solid phases is as determined by X-ray diffraction.